The invention addressed herein relates to a sensor unit according to the preamble of claim 1.
The following techniques are used in the semiconductor industry for production of semiconductor components: chemical vapor deposition (CVD), physical vapor deposition (PVD), implanting and (dry) etching processes. Typical pressure ranges for the used processes lie e.g. in the range of 10−4 to 10 mbar. Thereby e.g. sensor units with a capacitive membrane measuring cell are employed.
Especially with so-called ALD—(Atomic Layer Deposition) processes pressure measurements need to be carried out at temperatures of 300° Celsius or higher.
A sensor unit with a capacitive membrane measuring cell (CDG—Capacitance Diaphragm Gauge) is based on the elastic deformation of a thin membrane that is suspended above a massive body and hence separates two spaces from each other. A pressure difference between these spaces leads to a bending (deflection) of the membrane, whereby the membrane at a high pressure difference deflects stronger than at a low pressure difference. Metallic electrodes are provided in the region of the gap on the membrane and on the base body that is opposite to the membrane in order to form a capacitor. Naturally, the capacitance of the capacitor is dependent on the distance between the membrane and the base body. The change of capacitance of this capacitor is therefore a measure for the change of pressure. Sensor units of this type are known and described e.g. in WO 2007/019714 A1.
The relatively high temperatures used in the mentioned processes lead to corresponding constructional measures at the sensor units. That way, on the one hand attention must be paid that the electronic components required for processing the measured values do not overheat due to their proximity to the measuring cell, on the other hand the sensor units should show a certain compactness for a simple handling. Furthermore it has to be considered that in many cases a heater is heating the measuring cell to a temperature that lies higher than a condensation temperature of involved substances of a vacuum process to be measured. The temperature of the measuring cell thereby lies for example at least 10° C. above the condensation temperature. The involved substances are often very aggressive and the heating is an effective measure for keeping the substances away from sensitive parts of the measuring cell. With that it can be achieved that the measuring cell reliably works for a long period with high precision and high reproducibility.
Although the known sensor units preponderantly fulfill the previously explained requirements, yet it has turned out that the production of the known sensor units is elaborate and thus expensive.